I have to wonder whether the noise will ever lead to any specific component failure. Having owned cars with both M54 and N52 engines I have to say that the N52 is one nasty sounding motor full stop when compared to the M54 (and any other 6 cylinder unit I've ever owned).

The M54 sounds gutsy with smooth low frequency warm sounding bass tones, the N52 is a tweeter by comparison that screeches treble and cold rattly metallic tones - OK weird description but hopefully you get the jist.

Really hope this turns out well but sadly once confidence is lost it's almost impossible to recover completely even when you eventually get to the bottom of the issue (I had a nightmare with my X3 water pump and thermostat which dragged on for months and even though I know it's 100% sorted now I don't fully trust the car anymore and I'm always waiting for the next problem to appear - it hasn't but I can't stop feeling that way!).

Not sure I agree with that...I find the N52 smooth as silk and not in the least bit tinny or clattery. In fact I find it's bassy howl at 5k one of the sweetest engine noises ever. And thats coming from an E46 M3 owner. It's also one of the most reliable, once the water pump concern is addressed there are very few other problems it suffers from. It's one of bmw's best.

OK, so here is the update with my 2.5si, but long story short the problem remains. Aside from the noise when properly warm, the engine runs extremely sweetly, no hunting, engine wobble etc and no fault codes thrown. It’s extremely frustrating…

To recap.

Noise identified on the 6-7 mile journey home following purchase. Video earlier in post on page 3. Immediately raised with seller, James, who said he had not heard it previously.

Ran it to local specialist next day for their opinion – thought it was timing chain related and started quoting £1k+ fix but only guessing – no formal diagnosis but too busy to look at it.

Vehicle had oil and filter replacement within 100 miles of my purchase – James thought it could be related to this as he had read stories of bad filters etc but also thought it could be a heat shield.

James took vehicle back to replace oil and filter again. No change. Checked all heat shields etc. Then decided to look at both DISA valves and solenoids over the course of a Sunday – no issues found.

Went to see local BMW Master Tech to have a look – he thought it was timing chain related, or possibly a Vanos hub. Offered to diagnose but not a formal Indy business, has health issues and wanted to leave car on road overnight etc which I didn’t fancy.

Spoke to two separate Indy’s who were not keen on diagnosing over the phone and in honesty not keen full stop given the nature of the intermittent fault.

Eventually had a recommendation of an old school mechanic who would look at it. On the way to this garage I stopped at an Indy specialist in KL who I had spoken to over the phone. He had not heard the noise before (as everyone else had stated) and not generally seen issues on the 2.5si engine. He did not want to take the job on (too busy but also volunteered it would be a nightmare to diagnose). He thought it was timing chain related too based purely on where the noise appeared to be coming from but noises resonate and hard to pin point, especially if intermittent.

Car into old school mechanic. 40 years in game and BMW main dealer trained inc. Alpina vehicles with colleague 30+ years. They listened and thought it too was timing chain related although it was an odd noise and couldn’t understand why it was intermittent.

They have had the car eight weeks this Friday. We have communicated throughout the time they have had the car and despite hours and hours of work they are at the point of accepting defeat. First time for everything.

Initially we agreed to install a new chain tensioner as low cost item and existing item was slightly worn. No change following replacement, so agreed to do some investigative work.

They lifted the rocker cover to inspect and nothing visible. Timing chain and guides all looked good visually – camera used to check hidden areas where possible. Chain tension good.

Oil filter housing removed / checked following feedback from James and investigation online etc – mechanic was adamant there was nothing here that could be the cause but went through the process. Eventually worked to lower area of timing chain - sump removed and contaminant found in oil – small pieces. James has a contact that could assess this which was done after collecting a sample – it was deemed to be plastic, had not been through a combustion process etc. One other minor contaminant which was suggested to be silicone gasket related but barely traceable. James and his contact suggested it had to be chain guide related and it must be these that were worn. Fed info back to Colin who could see the logic but was reluctant to change them in all honesty but went with the analysis results and I agreed to proceed here.

New chain and guides fitted, along with the chain tensioner previously. Noise still there…. Old chain and guides show very little sign of wear apparently.

James called an Indy on south coast who suggested despite the change it could be pick-up pipe and strainer related as could be blocked. This was checked, cleaned etc and no issues found. Chain tension and oil to tensioner good.

You can imagine theirs and my own feelings at this stage…

Vehicle taken to main dealer in King’s Lynn where my mechanic had people he knows listen to the engine. Again, everyone drawing a blank. Had two Audi technicians he knows also see the vehicle and again, a blank. One suggested variable valve timing issue but my guy said the Audi VVT is different to that on a BMW etc and is adamant this is not the issue, well, as far as possible.

At this point, and probably a bit late down the road, he borrowed the chassis ears tool to try to pinpoint the problem as visually there is nothing to see. Noise appeared to be coming from the top of the timing chain area leading to potentially being a Vanos hub. Difficult to isolate, but thought it was on the exhaust side. Took second opinion from colleague who independently thought the same. Price of parts expensive and again he was reluctant to simply switch it out, but after much deliberation and based on early diagnosis by the BMW master tech I agreed to proceed. Exhaust side Vanos hub replaced. No fix…. Near death experience from stress and heart attack at both ends.

Re-check of sound, spoke to local indy specialists again who advised they would generally replace both hubs…. Not sure if this is correct or not in all honesty. Much deliberation again, both ends pulling hair out etc but agreed to second Vanos, intake side replaced – no fix!

So here we are. Colin is dumbfounded. His mates in the trade likewise.

Colin seems to think the noise has now moved down the rev range a little, coming in at c900 revs and disappearing at around 1200. That said, he said he has spent that much time listening to it at various times it was hard to be 100% sure on this point. His view is it sounds like something is catching. He also said the noise is there under load whilst increasing the revs from idle, but as soon as you come off load it disappears. It is hardly audible but he thinks it is there at idle when the engine is cold, it just gets louder when the engine is properly warm.

I am sure some of you must think Colin is incompetent. I think that would be unfair. He and his colleague have spent 20+ hours on my car, trying to fit it in around normal jobs to keep the price down. He is that embarrassed about it he is pretty much wiping 3/4 of his labour out, accepting I have to cover the parts replaced, themselves getting up to £1k. As he said, he was trying to help me out and in the end he has created a headache for himself.

James contacted me a bit out of the blue at lunchtime. Fair play for taking it upon himself to do so as no comms from me or chase up's. Must be something in the air having got the latest news from Colin this morning... Another one embarrassed and frustrated by the news… We should form a club. James today has made a good will gesture towards the costs incurred. I have no doubt he genuinely was unaware of the noise given his early actions to rectify things and the assistance he has provided, not to mention this gesture. It was a private sale and I am sure many out there offload problems to unsuspecting buyers and don't give a second thought...

So here we are.

Anyone got any bright ideas? Can it really be something simple we have all missed, including a number of specialists who have seen / heard it…. On the bright side I have just about forgotten what the Z looks like or drives like. I might just not pick it up...

Hindsight is a wonderful thing…. One to be philosophical about I guess.

And yes, I know it's not at the front of the engine, BUT it's connected to the crank, and therefore could be getting passed through to the timing chain.. Your more accurate description of the noise in above post, especially with it being so low down in the rev range got me thinking back to a Hyundai i30 (yes I know) that had a similar low rev noise. Was made more difficult to diagnose as it was a diesel, but I eventually traced it to the DMF using my chassis ears...

After all the work you have had done and the problem still their,I would drive the z to see if the noise increases,from what you say the car drives ok,with the noise only evident in a small band in the rev range.I feel for you with all this stress.

After all the work you have had done and the problem still their,I would drive the z to see if the noise increases,from what you say the car drives ok,with the noise only evident in a small band in the rev range.I feel for you with all this stress.

That would probably be my way of dealing with it. Chances are whatever it is will get louder and aid diagnosis..

Actually, no I'm lying. I hate being beaten, so would find out the cause.